Certain aspects of this disclosure relate to articles and/or assemblies having coated components or features, and processes for coating such articles. In particular, certain aspects of the disclosure relate to a metal article at least partially covered with one or more thermoset coatings and/or an inorganic coating capable of preventing galvanic corrosion from occurring when the article is in contact with a dissimilar metal or other material in the presence of an electrolyte, assemblies having such an article, and processes for producing such articles.
There are many different types of corrosion. In general, corrosion is the conversion of a material, for example, a metal, to a more stable form. There are, however, two major types of corrosion: general or uniform attack corrosion; and galvanic corrosion. General or uniform attack corrosion can occur, for example, when iron is in a wet or damp environment and it corrodes, forming iron oxide in the process.
Galvanic corrosion, on the other hand, occurs when two materials having different anodic indices or electro-potentials are in contact with, or close proximity to, one another in the presence of an electrolyte. The electro-potential difference produces electron flow between the materials. In such a system, one of the materials is more active (or less noble) and serves as an anode and the other material is less active (or more noble) and serves as a cathode. The anode corrodes at an accelerated rate, while the cathode corrodes at a lesser rate.
An example of a system in which galvanic corrosion can occur is a steel bolt securing a magnesium panel to an object in the presence of non-distilled water, such as a salt spray environment. The magnesium, being less noble than the steel, will corrode at an accelerated rate, while the steel will corrode at a slower rate. This problem is not limited to dissimilar metals, in that galvanic corrosion can occur when, for example, a steel bolt is used to secure a non-metal panel, such as a carbon fiber panel. In this system, the steel being less noble than the carbon fiber will corrode at an accelerated rate, while the carbon fiber panel will corrode at a slower rate. Again, when two materials having different anodic indices are in contact with or close proximity to one another, the potential for galvanic corrosion is present with the less noble material exhibiting accelerated corrosion.
Once an electrolyte is present, for example by the presence of non-distilled water, such as salt spray and the like, corrosion can occur, which can weaken the structural integrity of whatever material is acting as the anode by virtue of its relative electro-potential and/or result in an undesirable aesthetic appearance. Galvanic corrosion is a problem in the automotive and aerospace fields, amongst others.
In, for example, the automotive industry, there is a strong desire to reduce the weight of vehicles. Such light-weighting, is driven by the effort to increase fuel efficiency. As such, lighter weight materials, such as aluminum, magnesium and carbon fiber, are used in body and drive train components. However, in many instances, the use of light-weight components cannot be carried over to fasteners, such as bolts and the like. Thus, the bolts used are typically iron alloy materials, such as steel. The reluctance to use these light-weight materials in fasteners is due to their increased cost and the acceptance of steel fasteners, their strength and overall mechanical properties.
To prevent galvanic corrosion, similar materials or different materials with similar electro-potentials (anodic indices) can be used. However, this limits the types of combinations of materials available for the desired application.
In another scenario, a barrier can be imposed between the dissimilar materials. For example, a bolt coated with a polymeric material such as nylon or a polymeric seal can be disposed between the head of a bolt and the panel. However, nylon coatings or seals may not discourage galvanic corrosion, and may not meet the general mechanical requirements of the system. For example, the coating may be too thick and interfere with the engagement of the bolt with a female member (e.g., a nut), or increase the coefficient of friction when driving the bolt, or the resiliency of the coating or seal may result in the loss of tension when subject to temperature changes, e.g., heat up and cool down, of the system. In addition, such polymeric coatings or seals may not provide the barrier needed to prevent electron transfer between the dissimilar materials. Furthermore, resilient polymeric material may not maintain structural integrity with temperature fluctuations, vibrations and other forces to which the system may be subjected.
Accordingly, there is a need for a method for preventing galvanic corrosion in systems that have dissimilar materials in contact with or in close proximity to one another in the presence of an electrolyte. Desirably, such a method uses materials that withstand heat up and cool down cycles of the system while maintaining protection of the materials from galvanic corrosion. More desirably still, such a method uses materials that maintain the mechanical properties and requirements of the system. Still more desirably, such a method can be carried out in a manufacturing environment, in a cost effective manner.
There is also a need for a multi-part article system that exhibits resistance to galvanic corrosion under a variety of adverse environmental conditions, while maintaining the required mechanical properties, conditions, characteristics and specifications of the system.